Gas turbine engines include generally a rotor assembly and stationary mounting. The rotor assembly includes a number of components which rotate as a unit. For example, such components include a central shaft, shaft cones, compressor blades and disks, turbine buckets and wheels, and dynamic air seals. Each component is reacted upon by static and/or dynamic axial pressure forces. The vector sum of these forces is a net axial force or thrust generally in either the forward or aft direction. This net thrust tends to separate the rotor assembly from the stationary mounting.
In order to absorb this load without interfering with the free rotation of the rotor assembly a thrust bearing is employed. Typically, such thrust bearings are ball bearings encased within a thrust bearing housing. The load on thrust bearings varies as the pressures on the various rotor parts change. This problem is particularly acute in variable cycle engines where rotor thrust pressures can fluctuate widely. Net rotor axial thrust in the range of 0 to 20,000 pounds (force) is not uncommon. Moreover, under certain conditions net rotor axial thrust may change direction, a condition known as "cross-over." If net axial thrust is excessive, undue wearing and premature failure of thrust bearings may occur. If cross-over occurs, radial movement of the rotor may adversely affect seal clearances resulting in deterioration of engine operating characteristics.
In the past, one means to compensate for high net axial thrust has been to create regions in the engine where air pressure is used to balance net rotor thrust pressure. One such region exists behind the stator exit at compressor discharge. At the junction where high pressure air leaves the compressor and enters into the combustion chamber a seal is provided to reduce air lost. This seal may be located radially as desired. The region between the gas flowpath and the seal location is axially bounded by the forward positioned rotor and aft positioned stationary support structure. High pressure in this region, therefore, provides only forward thrust on the rotor. This region is known as a free balance area because the radial position of the seal determines the area of rotor surface on which this high pressure may act. For instance, decreasing the diameter of this seal increases rotor area thereby increasing forward rotor thrust.
In the past, designers have used the free balance area to balance rotor thrust so as to reduce the load on the ball thrust bearing. Two problems exist with this form of rotor balancing. First, this fixes the diameter of the compressor discharge seal to a predetermined value. Ideally, the diameter of this seal should be as small as possible so as to minimize the leakage around the seal. Such leakage is undesirable since it robs the system of high pressure air thereby decreasing engine efficiency. Second, a fixed free balance area is unable to compensate for variations in net rotor thrust.